Solar cell using low iron high transmission glass with antimony and corresponding method

ABSTRACT

A high transmission and low iron glass is provided for use in a solar cell. The glass substrate may be patterned on at least one surface thereof. Antimony (Sb) is used in the glass to improve stability of the solar performance of the glass upon exposure to ultraviolet (UV) radiation and/or sunlight. The combination of low iron content, antimony, and/or the patterning of the glass substrate results in a substrate with high visible transmission and excellent light refracting characteristics.

This application is a divisional of application Ser. No. 11/122,218,filed May 5, 2005 now U.S. Pat. No. 7,700,870, the entire disclosure ofwhich is hereby incorporated herein by reference in this application.

This invention relates to a high transmission low iron glass, includingantimony, for use in solar cells or the like. A method is also provided.In certain example embodiments, the glass composition used for the glassis a low-iron type glass composition which includes antimony. The glasssubstrate used in a solar cell may be patterned in certain exampleembodiments of this invention.

BACKGROUND AND SUMMARY OF EXAMPLE EMBODIMENTS OF THE INVENTION

Solar cells are known in the art. A solar cell may include, for example,a photoelectric transfer film made up of one or more layers locatedbetween a pair of substrate. These layers may be supported by a glasssubstrate. Example solar cells are disclosed in U.S. Pat. Nos.4,510,344, 4,806,436, 6,506,622, 5,977,477, and JP 07-122764, thedisclosures of which are hereby incorporated herein by reference.

Substrate(s), sometimes called superstrate(s), in a solar cell aresometimes made of glass. Glass that is fairly clear in color and highlytransmissive to visible light is sometimes desirable. Glass rawmaterials (e.g., silica sand, soda ash, dolomite, and/or limestone)typically include certain impurities such as iron, which is a colorant.The total amount of iron present is expressed herein in terms of Fe₂O₃in accordance with standard practice. However, typically, not all ironis in the from of Fe₂O₃. Instead, iron is usually present in both theferrous state (Fe²⁺; expressed herein as FeO, even though all ferrousstate iron in the glass may not be in the form of FeO) and the ferricstate (Fe³⁺). Iron in the ferrous state (Fe²⁺; FeO) is a blue-greencolorant, while iron in the ferric state (Fe³⁺) is a yellow-greencolorant. The blue-green colorant of ferrous iron (Fe²⁺; FeO) is ofparticular concern when seeking to achieve a fairly clear or neutralcolored glass, since as a strong colorant it introduces significantcolor into the glass. While iron in the ferric state (Fe³⁺) is also acolorant, it is of less concern when seeking to achieve a glass fairlyclear in color since iron in the ferric state tends to be weaker as acolorant than its ferrous state counterpart.

It has been found that the use of a low-iron highly transparent(optionally patterned) glass is advantageous for solar cellapplications. The use of the low-iron composition in combination withthe patterned surface(s) of the glass substrate(s) has been found to beadvantageous with respect to optical properties, thereby leading toincreased solar efficiency of a solar cell.

In certain example embodiments of this invention, a solar cell glasssubstrate has a visible transmission of at least 75% (more preferably atleast 80%, even more preferably at least 85%, and most preferably atleast about 90%). In making such a glass, a batch therefor includes abase glass (e.g., soda lime silica glass) and in addition comprises (orconsists essentially of in certain other embodiments) a very smallamount of total iron.

In the past some have tried to use cerium oxide in glass for solar cellapplications as an oxidizer. However, it has been found that the use ofsignificantly amounts of cerium oxide in solar cell glass can result ina loss of solar transmission after ultraviolet (UV) exposure, which isof course undesirable. Thus, in certain example embodiments of thisinvention, the use of cerium oxide is substantially avoided.

In this respect, it has surprisingly been found that the use of antimony(e.g., in the form of an oxide of antimony (Sb)) in high transmissionlow-iron glass for solar cells or the like results in a glass that neednot suffer from the aforesaid problem associated with cerium oxide.Accordingly in certain example embodiments of this invention, antimony(Sb) is provided in low-iron high transmission glass. Thus, theresulting glass may include antimony (Sb) and be substantially free ofcerium oxide so as to realize good stability of solar performance (e.g.,no or reduced loss of total solar transmission after UV or sunlightexposure).

In certain example embodiments, the patterned glass substrate may havefairly clear color that may be slightly yellowish (a positive b* valueis indicative of yellowish color). For example, in certain exampleembodiments, the patterned glass substrate may be characterized by avisible transmission of at least 90%, a total solar/energy value of atleast 90%, a transmissive a* color value of from −1.0 to +1.0 (morepreferably from −0.5 to +0.5, and most preferably from −0.2 to 0), and atransmissive b* color value of from 0 to +1.5 (more preferably from +0.1to +1.0, and most preferably from +0.2 to +0.7). These properties may berealized at an example non-limiting reference glass thickness of fromabout 3-4 mm.

In certain example embodiments of this invention, in combination withthe use of antimony (Sb), the glass has no more than 0.07% cerium oxide,more preferably no more than 0.06%, even more preferably no more than0.04% cerium oxide, even more preferably no more than 0.02% ceriumoxide, and possibly 0 or 0.01% cerium oxide.

In certain example embodiments of this invention, there is provided asolar cell comprising: a patterned glass substrate, wherein at least onesurface of the patterned glass substrate has a surface roughness of fromabout 0.1 to 1.5 μm; first and second conductive layers with at least aphotoelectric film provided therebetween; wherein the glass substrate isof a composition comprising:

Ingredient wt. % SiO₂ 67-75% Na₂O 10-20% CaO 5-15% total iron (expressedas Fe₂O₃) 0.001 to 0.06% cerium oxide 0 to 0.07% antimony oxide 0.01 to1.0%wherein the glass substrate has visible transmission of at least 90%, atransmissive a* color value of −1.0 to +1.0 and a transmissive b* colorvalue of from 0 to +1.5.

In other example embodiments of this invention, there is provided solarcell comprising: a glass substrate; a photoelectric film supported by atleast the glass substrate; wherein the glass substrate is of acomposition comprising:

Ingredient wt. % total iron (expressed as Fe₂O₃) 0.01 to 0.06% antimonyoxide  0.01 to 0.5%wherein the glass substrate has visible transmission of at least 90%, atransmissive a* color value of −1.0 to +1.0 and a positive transmissiveb* color value.

In other example embodiments of this invention, there is provided aglass substrate comprising:

Ingredient wt. % SiO₂ 67-75% Na₂O 10-20% CaO 5-15% total iron (expressedas Fe₂O₃) 0.001 to 0.06% cerium oxide 0 to 0.07% antimony oxide 0.01 to1.0%wherein the glass substrate has visible transmission of at least 90%, atransmissive a* color value of −1.0 to +1.0 and a transmissive b* colorvalue of from 0 to +1.5.

In still further example embodiments of this invention, there isprovided a method of making patterned glass, the method comprising:providing a molten glass batch in a furnace or melter comprising from67-75% SiO₂, from about 0.01 to 0.06% total iron, and antimony oxide;forwarding a glass ribbon from the furnace or melter to a nip betweenfirst and second rollers, at least one of the rollers having patterdefined in a surface thereof, wherein the glass ribbon reaches the nipat a temperature of from about 1,900 to 2,400 degrees F.; at the nip,transferring the pattern from the roller(s) to the glass ribbon; theglass ribbon being at a temperature of from about 1,100 to 1,600 degreesF. upon exiting the nip; annealing the glass ribbon at least after theribbon exits the nip, thereby providing a patterned glass having avisible transmission of at least 90%, from about 0.01 to 0.06% totaliron, and from about 0.01 to 1.0% antimony oxide.

IN THE DRAWINGS

FIG. 1 is a cross sectional view of a solar cell according to an exampleembodiment of this invention.

DETAILED DESCRIPTION OF CERTAIN EXAMPLE EMBODIMENTS OF THIS INVENTION

An example solar cell is illustrated in cross section in FIG. 1. Thesolar cell includes, for example and without limitation, hightransmission glass substrate 1, conductive film 2 which may betransparent, a photoelectric transfer film 3 which may include one ormore layers, a rear surface electrode 4, and an optional reflector 5. Incertain example embodiments, the photoelectric transfer film 3 mayinclude a p-type silicon inclusive layer, an i-type silicon inclusivelayer, and an n-type silicon inclusive layer. These silicon inclusivelayers may be composed of amorphous silicon or any other suitable typeof semiconductor with suitable dopants in certain example embodiments ofthis invention. The electrodes 2, 4 may be of a transparent conductorsuch as zinc oxide, or any other suitable material in certain exampleembodiments of this invention, and the reflector 5 may be of aluminum,silver or the like.

In certain example embodiments of this invention, one or both majorsurfaces (e.g., the interior surface only) of the glass substrate 1 maybe patterned. Light tends to be refracted at interface(s) resulting fromthe patterning of the glass substrate 1, thereby causing light toproceed through the semiconductor layer(s) at an angle(s) such that thepath is longer. As a result, more light can be absorbed by the solarcell and output current and/or efficiency can be improved/increased. Incertain example embodiments of this invention, the patterned surface(s)of the glass substrate 1 may have a surface roughness (betweenpeaks/valleys) of from about 0.1 to 1.5 μm, more preferably from about0.5 to 1.5 μm. In certain example embodiments of this invention, theglass substrate 1 has one or more surfaces which are patterned so as tohave a waviness feature defined therein. In the FIG. 1 embodiment, onlyone surface of the glass substrate 1 is patterned, although in otherexample embodiments both surfaces of the glass substrate may bepatterned.

The optional patterning is preferably defined in the glass substrate 1during the process of making the glass. An example technique for makingsuch patterned glass is as follows. A furnace or melter is provided, asare first and second opposing rollers which define a nip therebetween.At least one of the rollers has a pattern defined in a surface thereof,where the pattern is made up of a plurality of peaks and valleys. Aribbon of glass exiting the furnace or melter is fed into the nipbetween the patterning rollers and reaches the nip at a temperature offrom about 1,900 to 2,400 degrees F. At the nip, the pattern(s) from theroller(s) is transferred to the ribbon of glass, and then the patternedglass ribbon exits the nip at a temperature of from about 1,100 to 1,600degrees F. After leaving the nip, the patterned glass ribbon isannealed, and may then be cut into a plurality of sheets. These glasssheets may or may not be heat treated (e.g., thermally tempered), andmay be used in solar cell applications such as shown in FIG. 1. Exampletechniques for making the patterned glass substrate 1 are illustratedand described in U.S. Pat. Nos. 6,796,146 and/or 6,372,327 (except thatdifferent types of patterns are used), the disclosures of which arehereby incorporated herein by reference.

Certain glasses for patterned substrate 1 according to exampleembodiments of this invention utilize soda-lime-silica flat glass astheir base composition/glass. In addition to base composition/glass, acolorant portion may be provided in order to achieve a glass that isfairly clear in color and/or has a high visible transmission. Anexemplary soda-lime-silica base glass according to certain embodimentsof this invention, on a weight percentage basis, includes the followingbasic ingredients:

TABLE 1 EXAMPLE BASE GLASS Ingredient Wt. % SiO₂ 67-75%  Na₂O 10-20% CaO 5-15%  MgO 0-7% Al₂O₃ 0-5% K₂O 0-5%Other minor ingredients, including various conventional refining aids,such as SO₃, carbon, and the like may also be included in the baseglass. In certain embodiments, for example, glass herein may be madefrom batch raw materials silica sand, soda ash, dolomite, limestone,with the use of sulfate salts such as salt cake (Na₂SO₄) and/or Epsomsalt (MgSO₄×7H₂O) and/or gypsum (e.g., about a 1:1 combination of any)as refining agents. In certain example embodiments, soda-lime-silicabased glasses herein include by weight from about 10-15% Na₂O and fromabout 6-12% CaO.

In addition to the base glass (e.g., see Table 1 above), in making glassaccording to certain example embodiments of the instant invention theglass batch includes materials (including colorants and/or oxidizers)which cause the resulting glass to be fairly neutral in color (slightlyyellow in certain example embodiments, indicated by a positive b* value)and/or have a high visible light transmission. These materials mayeither be present in the raw materials (e.g., small amounts of iron), ormay be added to the base glass materials in the batch (e.g., antimonyand/or the like). In certain example embodiments of this invention, theresulting glass has visible transmission of at least 75%, morepreferably at least 80%, even more preferably of at least 85%, and mostpreferably of at least about 90% (sometimes at least 91%) (Lt D65). Incertain example non-limiting instances, such high transmissions may beachieved at a reference glass thickness of about 3 to 4 mm

In certain embodiments of this invention, in addition to the base glass,the glass and/or glass batch comprises or consists essentially ofmaterials as set forth in Table 2 below (in terms of weight percentageof the total glass composition):

TABLE 2 EXAMPLE ADDITIONAL MATERIALS IN GLASS General More MostIngredient (Wt. %) Preferred Preferred total iron 0.001-0.06% 0.005-0.045%   0.01-0.03% (expressed as Fe₂O₃): % FeO: 0-0.0040%0-0.0030% 0.001-0.0025%   glass redox <=0.10 <=0.06 <=0.04 (FeO/totaliron): cerium  0-0.07%  0-0.04%   0-0.02% oxide: antimony  0.01-1.0% 0.01-0.5%  0.1-0.3% oxide: SO₃:   0.1-1.0%  0.2-0.6%  0.25-0.5% TiO₂  0-1.0% 0.005-0.4%  0.01-0.04%

In certain example embodiments, the antimony may be added to the glassbatch in the form of one or more of Sb₂O₃ and/or NaSbO₃. Note alsoSb(Sb₂O₅). The use of the term antimony oxide herein means antimony inany possible oxidation state, and is not intended to be limiting to anyparticular stoichiometry.

In certain preferred embodiments, there is no cerium oxide in the glass.In particular, the presence of cerium oxide can have a detrimentaleffect on the transmission of the glass after exposure to UV and/orsunlight. This has been seen at 0.01 and 0.02% by weight. Thus, incertain example embodiments, the glass contains no cerium oxide. Incertain embodiments, the resulting glass may contain from 0 to 0.01% byweight of cerium oxide.

The low glass redox evidences the highly oxidized nature of the glass.Due to the antimony (Sb), the glass is oxidized to a very low ferrouscontent (% FeO) by combinational oxidation with antimony in the form ofantimony trioxide (Sb₂O₃), sodium antimonite (NaSbO₃), sodiumpyroantimonate (Sb(Sb₂O₅)), sodium or potassium nitrate and/or sodiumsulfate. In certain example embodiments, the composition of the glasssubstrate 1 includes at least twice as much antimony oxide as total ironoxide, by weight, more preferably at least about three times as much,and most preferably at least about four times as much antimony oxide astotal iron oxide.

In certain example embodiments of this invention, the colorant portionis substantially free of other colorants (other than potentially traceamounts). However, it should be appreciated that amounts of othermaterials (e.g., refining aids, melting aids, colorants and/orimpurities) may be present in the glass in certain other embodiments ofthis invention without taking away from the purpose(s) and/or goal(s) ofthe instant invention. For instance, in certain example embodiments ofthis invention, the glass composition is substantially free of, or freeof, one, two, three, four or all of: erbium oxide, nickel oxide, cobaltoxide, neodymium oxide, chromium oxide, and selenium. The phrase“substantially free” means no more than 2 ppm and possibly as low as 0ppm of the element or material.

The total amount of iron present in the glass batch and in the resultingglass, i.e., in the colorant portion thereof, is expressed herein interms of Fe₂O₃ in accordance with standard practice. This, however, doesnot imply that all iron is actually in the form of Fe₂O₃ (see discussionabove in this regard). Likewise, the amount of iron in the ferrous state(Fe⁺²) is reported herein as FeO, even though all ferrous state iron inthe glass batch or glass may not be in the form of FeO. As mentionedabove, iron in the ferrous state (Fe²⁺; FeO) is a blue-green colorant,while iron in the ferric state (Fe³⁺) is a yellow-green colorant; andthe blue-green colorant of ferrous iron is of particular concern, sinceas a strong colorant it introduces significant color into the glasswhich can sometimes be undesirable when seeking to achieve a neutral orclear color.

The use of antimony (e.g., in the form of antimony oxide) as an oxidizerin the glass batch acts as a decolorizer since during melting of theglass batch it causes iron in the ferrous state (Fe²⁺; FeO) to oxidizeto the ferric state (Fe³⁺). This role of antimony as an oxidizerdecreases the amount of ferrous state iron left in the resulting glass.The presence of antimony oxide in the glass batch causes an amount ofthe strong blue-green colorant of ferrous iron (Fe²⁺; FeO) to oxidizeinto the weaker yellow-green ferric iron colorant (Fe³⁺) during theglass melt (note: some ferrous state iron will usually remain in theresulting glass). The aforesaid oxidation of the iron tends to reducecoloration of the glass and also causes visible transmission toincrease. Any yellowish color caused by oxidation of iron into ferricstate (Fe³⁺) iron (i.e., positive b*) is acceptable in solar cellapplications and need not be compensated for by addition of othercolorants thereby saving cost in certain example embodiments of thisinvention.

It will be appreciated by those skilled in the art that the addition ofantimony oxide results in a glass with a lower “redox” value (i.e., lessiron in the ferrous state FeO). In this regard, the proportion of thetotal iron in the ferrous state (FeO) is used to determine the redoxstate of the glass, and redox is expressed as the ratio FeO/Fe₂O₃, whichis the weight percentage (%) of iron in the ferrous state (FeO) dividedby the weight percentage (%) of total iron (expressed as Fe₂O₃) in theresulting glass. Due to at least the presence of the antimony oxide, theredox of glass according to certain example embodiments of thisinvention is very low as mentioned above, and the amount of iron in theferrous state (FeO) will also be low as discussed above.

It is noted that glass according to certain example embodiments of thisinvention is often made via the known float process in which a tin bathis utilized. It will thus be appreciated by those skilled in the artthat as a result of forming the glass on molten tin in certain exemplaryembodiments, small amounts of tin or tin oxide may migrate into surfaceareas of the glass on the side that was in contact with the tin bathduring manufacture (i.e., typically, float glass may have a tin oxideconcentration of 0.05% or more (wt.) in the first few microns below thesurface that was in contact with the tin bath).

In view of the above, glasses according to certain example embodimentsof this invention achieve a neutral or substantially clear color and/orhigh visible transmission. In certain embodiments, resulting glassesaccording to certain example embodiments of this invention may becharacterized by one or more of the following transmissive optical orcolor characteristics when measured at a thickness of from about 1 mm-6mm (most preferably a thickness of about 3-4 mm; this is a non-limitingthickness used for purposes of reference only) (Lta is visibletransmission %). It is noted that in the table below the a* and b* colorvalues are determined per Ill. D65, 10 degree Obs.

TABLE 3 GLASS CHARACTERISTICS OF EXAMPLE EMBODIMENTS CharacteristicGeneral More Preferred Most Preferred Lta (Lt D65): >=85% >=90% >=91% %τe (ISO 9050): >=85% >=90% >=91% % FeO (wt. %): <=0.004%   <=0.003%  <=0.0020%    L* (Ill. D65, 90-99 n/a n/a 10 deg.): a* (Ill. D65, −1.0 to+1.0 −0.5 to +0.5 −0.2 to 0.0   10 deg.): b* (Ill. D65,   0 to +1.5 +0.1to +1.0 +0.2 to +0.7 10 deg.):

The aforesaid characteristics of the glass substrate 1 are for the glasssubstrate alone, not the overall solar cell or solar cell module.

As can be seen from Table 3 above, glasses for substrate 1 of certainembodiments of this invention achieve desired features of fairly clearcolor and/or high visible transmission, with slightly positive b* colorin certain embodiments, while not requiring iron to be eliminated fromthe glass composition. This may be achieved through the provision of theunique material combinations described herein.

Examples 1-2

Example glasses for substrates 1 were made and tested according toexample embodiments of this invention. Glasses of this invention may bemade from batch ingredients using well known glass melting and refiningtechniques. The compositions of the glasses according to the examplesare set forth below. All amounts of ingredients are in terms of weightpercentage.

TABLE 4 EXAMPLES Compound Ex. 1 Ex. 2 SiO₂: 71.78 71.21 Na₂O: 13.5913.71 CaO: 9.23 9.69 MgO 4.07 4.06 Al₂O₃: 0.59 0.8 K₂O: 0.28 0.03 SO₃:0.416 0.437 TiO₂: 0.012 0.012 Fe₂O₃ (total iron): 0.027 0.024 Ceriumoxide: 0 0 Cr₂O₃: 0.0008 0.0008 Sb₂O₃: 0.2 0.2 Glass Redox: 0.04 0.025

Solar characteristics for the resulting Example glasses were as followsin the table below, with the below measurements taken after the melt andformation of the glass. It is noted that Lta (visible transmission %)was measured in accordance with Ill. D65, τe (total energy or totalsolar) was measured in accordance with ISO 9050 (incorporated herein byreference), and transmissive L*, a* and b* color coordinates (CIE) weremeasured using Ill. D65, 10 degree observer. All samples were from 3-4mm thick.

Characteristics of Examples 1-2

TABLE 4 Characteristic Ex. 1 Ex. 2 % Lta 91.48 91.65 % τe 91.35 91.53Fe₂O₃ (total iron %): 0.027 0.024 FeO (wt. %) 0.0011 0.0006 L* 96.6796.72 a* −0.08 −0.04 b* +0.41 +0.38 Glass Redox: 0.04 0.025

Once given the above disclosure many other features, modifications andimprovements will become apparent to the skilled artisan. Such features,modifications and improvements are therefore considered to be a part ofthis invention, the scope of which is to be determined by the followingclaims:

What is claimed is:
 1. A method of making patterned glass, the methodcomprising: providing a molten glass batch in a furnace or meltercomprising from 67-75% SiO₂, from about 0.01 to 0.06% total iron, andantimony oxide; forwarding a glass ribbon from the furnace or melter toa nip between first and second rollers, at least one of the rollershaving patter defined in a surface thereof, wherein the glass ribbonreaches the nip at a temperature of from about 1,900 to 2,400 degreesF.; at the nip, transferring the pattern from the roller(s) to the glassribbon; the glass ribbon being at a temperature of from about 1,100 to1,600 degrees F. upon exiting the nip; annealing the glass ribbon atleast after the ribbon exits the nip, thereby providing a patternedglass having a visible transmission of at least 90%, from about 0.01 to0.06% total iron, and from about 0.01 to 1.0% antimony oxide, whereinthe patterned glass has a glass redox of <=0.10.
 2. The method of claim1, wherein the patterned glass has no more than 0.04% cerium oxide, atransmissive a* color value of −0.5 to +0.5 and a transmissive b* colorvalue of from +0.1 to +1.0.
 3. The method of claim 1, wherein thepatterned glass has a transmissive a* color value of −1.0 to +1.0, and atransmissive b* color value of from 0 to +1.5.
 4. The method of claim 1,wherein the patterned glass has a transmissive a* color value of −1.0 to+1.0.